1. Field of the Invention
This invention relates to hot-cathode low pressure rare gas filled discharge fluorescent lamps which are used for office-automation apparatus such as copying machines and facsimile equipment.
2. Description of the Prior Art
Recently fluorescent lamps which utilize illumination from rare gas discharge have been used for luminous source of office automation apparatus. An example of such lamps is high luminous glow discharge lamps described in "Toshiba Review" Vol. 40, No. 12, pages 1079 to 1082. The lamp is a cold-cathode rare gas filled fluorescent lamp in which a gas comprising Xe as the main component is filled in this tube, and fluorescent material is excited with ultra-violet rays generated from glow discharge to illuminate. Because the lamp utilizes no mercury, it exhibits stable illumination output over the wide temperature range, and illumination color can be varied by selecting fluorescent material. However the cold-cathode rare gas filled discharge lamp requires a high voltage to start, this is a problem to use this type of lamp. The inventors of the present invention have worked for development of rare gas filled fluorescent lamps suitably used for office automation apparatus having not only a low starting voltage to reduce problems of high voltage by providing hot-cathode electrodes, but also low dependency on temperature and short response time performance, which are advantages of rare gas filled discharge fluorescent lamps, and thus attained to the discharge fluorescent lamps filled with low pressure rare gas having the desired performances.
However a new problem of the hot-cathode discharge fluorescent lamps filled with low pressure rare gas having above mentioned constitution was found, that is, the luminous maintenance deteriorates rapidly as the tube diameter is decreased below 20 mm, wherein such diameter is usually desirable for luminous source of office automation apparatus.
It was found by the inventors that the deterioration of luminous maintenance is caused from ion bombardment of positive column. That is, in a hot-cathode lamp with a thin diameter, the distance between the center axis of positive column and fluorescent material layer is short, particularly in a tube with a diameter of 20 mm or less, hence the fluorescent material deteriorates severely due to the effect of ion bombardment comparing with cold-cathode type lamps, thus this is the cause of rapid deterioration of luminous maintenance.